Thickness-dependent third-order optical nonlinearity in indium tin oxide films

We investigate the third-order optical nonlinearity of indium tin oxide (ITO) films of different thicknesses prepared by the magnetron sputtering technique at [Formula: see text]C. The third-order optical nonlinearity in ITO films was found to increase with a decrease in thickness. Specifically, the nonlinear refractive index [Formula: see text] and nonlinear absorption coefficient [Formula: see text] in the ITO film of a thickness of 20[Formula: see text]nm were around ten times larger than those in the 200[Formula: see text]nm-thick ITO film. Such changes of third-order optical nonlinearity in ITO films can be explained by orbital theory. The sp- and [Formula: see text]orbital in In–O bonds both contribute to the optical nonlinearity, considering the length of In–O bonds. As the thickness of the thin film increases, the interstitial oxygen in the atom structure reduces, and the polarization of the In–O bonds decreases due to the less asymmetric distortion, leading to a weakening of the optical nonlinearity. The results have an important significance for the understanding and the modulation of the optical nonlinearity in ITO films.